Union officials believe the explosion which rocked the BW Offshore-contracted Cidade de Sao Mateus floating oil production, storage and offloading vessel on Wednesday was caused by a gas leak.
BWO has confirmed that there were five fatalities, up from the three known on the first day of the disaster, while four of the 74 crew on board at the time are still missing.
While 10 injured workers were quickly flown off the FPSO, BWO said two of them were in critical condition.
Search and rescue operations were carried out into the night.
"This is a tragic day, and our primary focus now is on the crew and their families," BWO CEO Carl Arnet said.
"We cannot rest until the last four of our men are found. We express our gratitude to Petrobras and the Brazilian authorities for the tireless efforts in this time and wish to thank our peers and partners for their support."
The FPSO was about 50km offshore and produces oil and gas from the Camarupim and Camarupim Norte fields in Brazil's Espirito Santo Basin.
